Radio detection and ranging, commonly known as radar is used to detect and track a target object or objects. Radar systems typically emit electromagnetic energy and detect reflection of the emitted energy scattered by the target object. By analyzing time difference of arrival, Doppler shift and other changes in the reflected energy, the location and movement of the target object can be determined.
Phased array antenna systems employ a plurality of individual antenna elements or subarrays of antenna elements that are separately excited. Radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beam front or cumulative wave front of electromagnetic energy radiating from the array travels in a selected direction. Differences in phase or timing among the antenna activating signals determines the direction in which the cumulative beam from the antenna is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives. Such processing is well known to those of ordinary skill in the art.
Referring now to FIG. 1, there is shown a conventional radar apparatus 10. Radar apparatus 10 includes an Integrated Friend or Foe (IFF) or Secondary Surveillance Radar (SSR) antenna 150 physically bolted to the upper surface of a substantially planar Primary Surveillance Radar (PSR) antenna structure 100. The illustrated radar apparatus 10 shows an SSR/PSR arrangement for the AN/TPS-59 system manufactured by Lockheed Martin Corporation.
Where apparatus 10 is portable in nature, antenna structure 100 may fold—to facilitate stowage and transport. Referring now also to FIG. 2A, there is shown a schematic illustration of a folded PSR antenna structure, such as the AN/TPS-59 PSR antenna, in a deployed or operational configuration or state. FIG. 2B schematically depicts the same PSR antenna as shown in FIG. 2A in a folded configuration or state, so as to facilitate transport, for example. The illustrated PSR antenna 100 includes three modules or sections. The “wing” sections 10a, 10b of the linear arrays are vertically offset from the center section 10c and connected thereto by two rotary joints 90. The rotary joints 90 operate in conventional fashion to enable the “wing” sections to fold and interleave with the center section and provide an array in the transport configuration shown in FIG. 2B. The wing sections are sized and coupled via the rotary joints in an appropriate manner so that a slight gap (g) exists between the wing sections in transport mode, so as to avoid damage to the array. Septum 20 is similarly segmented in wing sections 20a, 20b and center section 20c and operates in similar manner. For the TPS-59 radar array in transport configuration, the width w′ (FIG. 2B) is less than 96 inches wide (in contrast to the width w in deployed mode, which is about 192 inches or 16 feet wide). PSR antenna structure 100 may be on the order of about 20 feet tall.
Where apparatus 10 is portable in nature, it is desired to be lightweight. Accordingly, light-weight materials are often utilized in antenna structure 100. Further, as can be seen in FIG. 1, antenna structure 100 has a significant cross-sectional area, as compared to the depth of antenna structure 100. Due, at least in part, to the lightweight nature of antenna structure 100, and its significant cross-section, antenna structure 100 may be prone to internal stresses that cause it to distort, e.g., bend or shift under torque. This may occur with or without external stimulus. For example, a portion of structure 100 may distort relative to another portion of structure 100 over time due to its shear size. Further, structure 100 may be particularly prone to distort when subjected to external loading, such as wind loads, heating loads resulting from solar radiation and/or vibration induced loads, all by way of non-limiting example only.
When structure 100 distorts, the individual antenna elements change their relative position—causing errors or even failures in their transmission/reception, such as undesirable side-lobes. Even a slight change in element orientation from the intended operating plane of antenna structure 100 is amplified significantly at the significant operating distances of radar antennas, e.g., 100 miles or more.
By way of further, non-limiting example only, a 3-piece radar structure, as is shown in FIGS. 2a, 2b, has transmitters in all 3 sections. The sections must maintain a substantially constant plane from initial deployment of the system. If any of the 3 pieces deviate from this plane, the resulting error will impact the accuracy of the system.
One possible cause of distortion of structure 100 may come from a combination of play in the hinges 90 and the deflection of the wing sections, which allows the wing sections to displace out of the plane of the center section. Another possible cause of distortion is when the center section is twisted out of it's original plane by external forces, e.g., wind loads.
Accordingly, it is desired to detect and/or mitigate undesired distortions in the structure of radar array antennas.